There are three things a gasoline engine needs: air, fuel and spark. The spark plug ignites the air/fuel mixture, producing the combustion that powers the engine. Despite the fact that spark plugs have featured in vehicles for more than a century, manufacturers are pushing back the technical boundaries. Still plugging away with just-auto’s components research, Matthew Beecham talked with Tim Howes, technical services manager, NGK Spark Plugs (UK) Ltd.
just-auto: Could we start by asking if you could summarise the factors which influence the design of a glow plug?
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Tim Howes: The volume of the combustion chamber is important as glow plugs are unsuitable for large capacity diesel engines of the type found in HGVs, plant and marine applications. The shape of the cylinder head and geometry of the combustion chamber will have a direct bearing on the dimensions of the glow plug metal shell. The prevailing emission regulations in the territory of operation will dictate the heating performance and characteristics so that smoke and other unwanted emissions are kept within limits under cold start conditions and, of course, cost to the OEM is undeniably a major factor.
What challenges can direct injection diesel systems pose for glow plugs?
The DI combustion chamber is already crowded with the fuel injector nozzle and large inlet and exhaust valves and the glow plug also has to be installed directly into the chamber avoiding these components. Indirect injection engines will have a pre-combustion or swirl chamber into which the glow plug protrudes without obstruction. Smaller diameter glow tubes mean that the combustion chamber walls are stronger which is also advantageous in the modern DI engine.
We’ve seen rapid start glow plugs emerge on the market. How do you see that technology evolving?
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By GlobalDataPre-heating times have been reduced from 30 seconds to less than two seconds, but it is difficult to envisage reducing those times significantly especially with other changes to CI engines on the horizon
In terms of technical specification of glow plugs, what else do you see happening? i.e. longer and thinner plugs?
That is exactly what has happened, much slimmer heater tubes, smaller thread diameters and longer bodies are the norm for many recent designs. Again, significant changes in the dimensions will become increasingly more challenging and probably unnecessary. Glow plugs can also be designed to function as pressure sensors to provide essential information to the management system.
What are the factors pushing innovations in spark plugs?
The demand for plugs with greater ignition performance, longer service life and reduced general dimensions continues apace. Ever leaner air/fuel mixtures and higher applied voltages create significant challenges for plug design. As with the modern high speed diesel engine, the spark ignition engine has seen the space occupied by the valves increase significantly, thus reducing the space available for the spark plug.
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